src/glsl/opt_discard_simplification.cpp

   1 /*
   2  * Copyright © 2010 Intel Corporation
   3  *
   4  * Permission is hereby granted, free of charge, to any person obtaining a
   5  * copy of this software and associated documentation files (the "Software"),
   6  * to deal in the Software without restriction, including without limitation
   7  * the rights to use, copy, modify, merge, publish, distribute, sublicense,
   8  * and/or sell copies of the Software, and to permit persons to whom the
   9  * Software is furnished to do so, subject to the following conditions:
  10  *
  11  * The above copyright notice and this permission notice (including the next
  12  * paragraph) shall be included in all copies or substantial portions of the
  13  * Software.
  14  *
  15  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  16  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  17  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
  18  * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  19  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
  20  * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
  21  * DEALINGS IN THE SOFTWARE.
  22  */
  23
  24 /**
  25  * \file opt_discard_simplification.cpp
  26  *
  27  * This pass simplifies if-statements and loops containing unconditional
  28  * discards.
  29  *
  30  * Case 1: Both branches contain unconditional discards:
  31  * -----------------------------------------------------
  32  *
  33  *    if (cond) {
  34  *       s1;
  35  *       discard;
  36  *       s2;
  37  *    } else {
  38  *       s3;
  39  *       discard;
  40  *       s4;
  41  *    }
  42  *
  43  * becomes:
  44  *
  45  *    discard
  46  *
  47  * Case 2: The "then" clause contains an unconditional discard:
  48  * ------------------------------------------------------------
  49  *
  50  *    if (cond) {
  51  *       s1;
  52  *       discard;
  53  *       s2;
  54  *    } else {
  55  *       s3;
  56  *    }
  57  *
  58  * becomes:
  59  *
  60  *    if (cond) {
  61  *       discard;
  62  *    } else {
  63  *       s3;
  64  *    }
  65  *
  66  * Case 3: The "else" clause contains an unconditional discard:
  67  * ------------------------------------------------------------
  68  *
  69  *    if (cond) {
  70  *       s1;
  71  *    } else {
  72  *       s2;
  73  *       discard;
  74  *       s3;
  75  *    }
  76  *
  77  * becomes:
  78  *
  79  *    if (cond) {
  80  *       s1;
  81  *    } else {
  82  *       discard;
  83  *    }
  84  */
  85
  86 #include "glsl_types.h"
  87 #include "ir.h"
  88
  89 class discard_simplifier : public ir_hierarchical_visitor {
  90 public:
  91    discard_simplifier()
  92    {
  93       this->progress = false;
  94    }
  95
  96    ir_visitor_status visit_enter(ir_if *);
  97    ir_visitor_status visit_enter(ir_loop *);
  98
  99    bool progress;
 100 };
 101
 102 static ir_discard *
 103 find_unconditional_discard(exec_list &instructions)
 104 {
 105    foreach_list(n, &instructions) {
 106       ir_discard *ir = ((ir_instruction *) n)->as_discard();
 107       if (ir != NULL && ir->condition == NULL)
 108          return ir;
 109    }
 110    return NULL;
 111 }
 112
 113 static bool
 114 is_only_instruction(ir_discard *discard)
 115 {
 116    return (discard->prev->is_head_sentinel() &&
 117            discard->next->is_tail_sentinel());
 118 }
 119
 120 ir_visitor_status
 121 discard_simplifier::visit_enter(ir_if *ir)
 122 {
 123    ir_discard *then_discard = find_unconditional_discard(ir->then_instructions);
 124    ir_discard *else_discard = find_unconditional_discard(ir->else_instructions);
 125
 126    if (then_discard == NULL && else_discard == NULL)
 127       return visit_continue;
 128
 129    /* If both branches result in discard, replace whole if with discard. */
 130    if (then_discard != NULL && else_discard != NULL) {
 131       this->progress = true;
 132       ir->replace_with(then_discard);
 133       return visit_continue_with_parent;
 134    }
 135
 136    /* Otherwise, one branch has a discard. */
 137    if (then_discard != NULL && !is_only_instruction(then_discard)) {
 138       this->progress = true;
 139       ir->then_instructions.make_empty();
 140       ir->then_instructions.push_tail(then_discard);
 141    } else if (else_discard != NULL && !is_only_instruction(else_discard)) {
 142       this->progress = true;
 143       ir->else_instructions.make_empty();
 144       ir->else_instructions.push_tail(else_discard);
 145    }
 146
 147    visit_list_elements(this, &ir->then_instructions);
 148    return visit_continue_with_parent;
 149 }
 150
 151 ir_visitor_status
 152 discard_simplifier::visit_enter(ir_loop *ir)
 153 {
 154    ir_discard *discard = find_unconditional_discard(ir->body_instructions);
 155
 156    if (discard) {
 157       ir->replace_with(discard);
 158       return visit_continue_with_parent;
 159    }
 160
 161    return visit_continue;
 162 }
 163
 164 bool
 165 do_discard_simplification(exec_list *instructions)
 166 {
 167    /* Look for a top-level unconditional discard */
 168    ir_discard *discard = find_unconditional_discard(*instructions);
 169    if (discard != NULL) {
 170       instructions->make_empty();
 171       instructions->push_tail(discard);
 172       return true;
 173    }
 174
 175    discard_simplifier v;
 176
 177    visit_list_elements(&v, instructions);
 178
 179    return v.progress;
 180 }